Reduced VOC emission alkenyl aromatic polymer foams and processes

ABSTRACT

A method for producing an alkenyl aromatic polymer foam structure that comprises melting at least one alkenyl aromatic polymer resin. An effective amount of blowing agent mixture is dissolved in the at least one alkenyl aromatic polymer. The blowing agent mixture comprises at least one VOC blowing agent and acetone. The majority by mole percent of the VOC blowing agent is selected from n-butane, iso-butane, propane and combinations thereof. An extrudate is formed comprising the at least one alkenyl aromatic polymer resin, the at least one VOC blowing agent, and acetone. The extrudate comprises less than about 2.7 wt. % of the at least one VOC blowing agent. The extrudate is transferred to an expansion zone and permitted to expand to produce the foam structure. The foam structure has an extruded density less than about 7 lbs/ft 3  and a basis weight of less than about 20 grams per 100 in 2 .

FIELD OF THE INVENTION

[0001] The present invention relates generally to foam processes forproducing an alkenyl aromatic polymer foam structure. More particularly,the present invention is directed to foam processes that produce analkenyl aromatic polymer foam structure using reduced amounts of VOCblowing agent mixtures.

BACKGROUND OF THE INVENTION

[0002] Foam, such as an alkenyl aromatic polymer foam (e.g., polystyrenefoam), may be made by combining a physical blowing agent with moltenpolystyrene resin under pressure and, after thorough mixing, extrudingthe combination through an appropriate die into a lower pressureatmosphere.

[0003] From about the 1950's to the present, physical blowing agents ofchoice have been halocarbons, hydrocarbons, or combinations thereof.Examples of these include commercially available halocarboncompositions, such as dichlorodifluoromethane andtrichlorofluoromethane, and the C₄-C₆ hydrocarbons. Hydrocarbons with atleast three carbon atoms are classified as volatile organic compounds(VOCs). These physical blowing agents, however, have disadvantagesbecause they are released into the atmosphere during and after foamproduction and, thus, are a source of pollution.

[0004] In the past, carbon dioxide has been experimented with as ablowing agent for resins, such as polystyrene resins. Carbon dioxide hasbeen listed as a blowing agent or as a co-blowing agent in thefollowing: U.S. Pat. No. 3,431,164 to Gilbert et al., U.S. Pat. No.4,198,363 to Noel, and U.S. Pat. No. 4,470,938 to Johnson. Carbondioxide has a disadvantage of a very high foaming rate caused by itshigh volatility. A very high foaming rate results in a high degree ofcorrugation in the extruded sheet. Because of the high degree ofcorrugation, a lower amount of blowing agent is used as compared tousing a conventional hydrocarbon or halocarbon blowing agent. When alower amount of blowing agent is used, higher viscosities and a loweroutput rate are obtained.

[0005] Carbon dioxide has been used as a co-blowing agent or a diluentfor hydrocarbon or halocarbon blowing agents. This is illustrated, forexample, in U.S. Pat. No. 4,344,710 to Johnson et al., and in an articleby L. M. Zwolinski and F. J. Dwyer, “Extruded Polystyrene Foam WithCFC/Carbon Dioxide Blowing Agents,” ANTEC '86, pages 30-33, ConferenceProceedings, Society of Plastic Engineers 44th Annual TechnicalConference and Exhibit. Because of the high volatility of carbondioxide, it can only be substituted at a low level in the process. Thus,the use of carbon dioxide as a co-blowing agent with hydrocarbon orhalocarbon blowing agents still results in a significant amount ofprocess emission that is a source of pollution.

[0006] Therefore, a need exists for a foam process that reduces fugitiveVOC emissions by reducing the concentration of the VOC blowing agent,while still producing a foam with desirable properties.

SUMMARY OF THE INVENTION

[0007] According to one method of producing an alkenyl aromatic polymerfoam structure, at least one alkenyl aromatic polymer resin is melted.An effective amount of blowing agent mixture is dissolved in the atleast one alkenyl aromatic polymer. The blowing agent mixture comprisesat least one volatile organic compound (VOC) blowing agent and acetone.The majority by mole percent of the VOC blowing agent is selected fromnormal butane (“n-butane”), iso-butane (“i-butane”), propane andcombinations thereof. An extrudate is formed comprising the at least onealkenyl aromatic polymer resin, the at least one VOC blowing agent, andacetone. The extrudate comprises less than about 2.7 wt. % of the atleast one VOC blowing agent. The extrudate is transferred to anexpansion zone and permitted to expand in the expansion zone to producethe alkenyl aromatic polymer foam structure. The alkenyl aromaticpolymer foam structure has an extruded density less than about 7 lbs/ft³and a basis weight of less than about 20 grams per 100 in².

[0008] According to one embodiment, an alkenyl aromatic polymer foamstructure is prepared by the process comprising melting at least onealkenyl aromatic polymer resin. An effective amount of blowing agentmixture is dissolved in the at least one alkenyl aromatic polymer. Theblowing agent mixture comprises at least one volatile organic compound(VOC) blowing agent and acetone. The majority by mole percent of the VOCblowing agent is selected from n-butane, iso-butane, propane andcombinations thereof. An extrudate is formed comprising the at least onealkenyl aromatic polymer resin, the at least one VOC blowing agent, andacetone. The extrudate comprises less than about 2.7 wt. % of the atleast one VOC blowing agent. The extrudate is transferred to anexpansion zone and permitted to expand in the expansion zone to producethe alkenyl aromatic polymer foam structure. The alkenyl aromaticpolymer foam structure has an extruded density less than about 7 lbs/ft³and a basis weight of less than about 20 grams per 100 in².

[0009] According to another method of producing an alkenyl aromaticpolymer foam structure, at least one alkenyl aromatic polymer resin ismelted. An effective amount of blowing agent mixture is dissolved in theat least one alkenyl aromatic polymer. The blowing agent mixtureconsists essentially of at least one volatile organic compound (VOC)blowing agent and acetone. The VOC blowing agent is selected fromn-butane, iso-butane, propane and combinations thereof. An extrudate isformed comprising the at least one alkenyl aromatic polymer resin, theat least one VOC blowing agent, and acetone. The extrudate comprisesless than about 2.7 wt. % of the at least one VOC blowing agent. Theextrudate is transferred to an expansion zone and permitted to expand inthe expansion zone to produce the alkenyl aromatic polymer foamstructure. The alkenyl aromatic polymer foam structure has an extrudeddensity less than about 7 lbs/ft³ and a basis weight of less than about20 grams per 100 in².

[0010] According to a further method of producing an alkenyl aromaticpolymer foam structure, at least one alkenyl aromatic polymer resin ismelted. An effective amount of blowing agent mixture is dissolved in theat least one alkenyl aromatic polymer. The blowing agent mixtureconsists essentially of at least one volatile organic compound (VOC)blowing agent, acetone and carbon dioxide. The VOC blowing agent isselected from n-butane, iso-butane, propane and combinations thereof. Anextrudate is formed comprising the at least one alkenyl aromatic polymerresin, the at least one VOC blowing agent, acetone and carbon dioxide.The extrudate comprises less than about 2.7 wt. % of the at least oneVOC blowing agent. The extrudate is transferred to an expansion zone andpermitted to expand in the expansion zone to produce the alkenylaromatic polymer foam structure. The alkenyl aromatic polymer foamstructure has an extruded density less than about 7 lbs/ft³ and a basisweight of less than about 20 grams per 100 in².

[0011] According to a yet another method for producing an alkenylaromatic polymer foam structure, at least one alkenyl aromatic polymerresin is melted. An effective amount of blowing agent mixture isdissolved in the at least one alkenyl aromatic polymer. The blowingagent mixture consists essentially of at least one volatile organiccompound (VOC) blowing agent, acetone and carbon dioxide. The VOCblowing agent is selected from n-butane, iso-butane, propane andcombinations thereof. An extrudate is formed comprising the at least onealkenyl aromatic polymer resin, the at least one VOC blowing agent,acetone and carbon dioxide. The extrudate comprises less than about 2.4wt. % of the at least one VOC blowing agent. The extrudate istransferred to an expansion zone and permitted to expand in theexpansion zone to produce the alkenyl aromatic polymer foam structure.The alkenyl aromatic polymer foam structure has an extruded density lessthan about 7 lbs/ft³ and a basis weight of less than about 20 grams per100 in².

[0012] According to one method of forming a thermoformed alkenylaromatic polymer foam article, at least one alkenyl aromatic polymerresin is melted. An effective amount of blowing agent mixture isdissolved in the at least one alkenyl aromatic polymer. The blowingagent mixture consists essentially of at least one volatile organiccompound (VOC) blowing agent and acetone. The majority by mole percentof the VOC blowing agent is selected from n-butane, iso-butane, propaneand combinations thereof. An extrudate is formed comprising the at leastone alkenyl aromatic polymer resin, the at least one VOC blowing agent,and acetone. The extrudate comprises less than about 2.7 wt. % of the atleast one VOC blowing agent. The extrudate is transferred to anexpansion zone and permitted to expand in the expansion zone to producethe alkenyl aromatic polymer foam structure. The alkenyl aromaticpolymer foam structure has a basis weight of less than about 20 gramsper 100 in². The alkenyl aromatic polymer foam structure is thermoformedinto an article, the thermoformed density of the article is less than 6lbs/ft³.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGURE is a schematic flow diagram of an overall sequence ofoperations involved in the manufacture of a foamed alkenyl aromaticpolymer sheet with a blowing agent mixture according to one embodimentof the present invention.

[0014] While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawing and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed but, on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0015] The processes of the present invention that form an alkenylaromatic polymer foam structure (e.g., polystyrene foam) employ ablowing agent mixture to achieve reduced fugitive VOC emissions.According to one method, the blowing agent mixture of the presentinvention comprises at least one volatile organic compound (VOC) blowingagent and acetone. The VOC blowing agent is selected from n-butane,iso-butane, propane and combinations thereof. The extrudate that isformed from at least one alkenyl aromatic polymer (e.g., polystyrene),the at least one VOC blowing agent, and acetone comprises less thanabout 2.7 wt. % of the at least one VOC blowing agent and is insubstantial absence of VOC blowing agents having at least five carbonatoms.

[0016] Alkenyl Aromatic Polymer

[0017] The term “alkenyl aromatic polymer” as used herein includespolymers of aromatic hydrocarbon molecules that contain an aryl groupjoined to a olefinic group with only double bonds in the linearstructure, such as styrene, α-methylstyrene, o-, m- and p-methylstyrene,α-ethylstyrene, o-, m-, p-ethylstyrene, 2,4-dimethylstyrene,α-vinylxylene, vinyl toluene and the like. Alkenyl aromatic polymersalso include homopolymers of styrene (commonly referred to aspolystyrene), copolymers of styrene (e.g., styrene-acrynitrilecopolymers, SAN), and rubber-toughened polystyrene (commonly referred toas high impact polystyrene, HIPS).

[0018] The term “polystyrene resin” or “polystyrenic resin” as usedherein includes styrene homopolymers and copolymers containing apredominant portion of styrene, i.e., greater than about 50 mol. %styrene, are preferred, with those containing greater than about 75 mol.% styrene being especially preferred. With respect to a styrenecopolymer, the comonomer may be any other ethylenically unsaturatedmaterial such as the conjugated 1,3-dienes, e.g., butadiene, isoprene,etc., alpha-beta-unsaturated monocarboxylic acids and derivativesthereof, e.g., acrylic acid, methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate, and the corresponding esters ofmethacrylic acid, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, etc.

[0019] The term “polystyrene resin” as used herein also includes blendsof at least 50 mol. % of the styrene homopolymer with anotherpredominately styrenic copolymer. The physical blends are combined indry form after the blends have previously been polymerized. The term“polystyrene resin” as used herein also includes blends of at least 50mol. % (preferably at least 75 mol. %) of a styrene homopolymer and/orcopolymer with other polymers. For example, blends of a styrene polymerwith polyphenylene oxide. Other contemplated blends include apolystyrene resin with a small amount of polyolefin (e.g., less thanabout 2 wt. % polyolefin).

[0020] Blowing Agent Mixture

[0021] According to one method, the blowing agent mixture or blend ofthe present invention comprises at least one volatile organic compound(VOC) blowing agent and acetone. The majority of the VOC blowing agentby mole percent is selected from n-butane, iso-butane, propane andcombinations thereof. It is preferable that the VOC blowing agentmixture is made in substantial absence of VOC blowing agents having atleast five carbon atoms and preferably made in the absence of VOCblowing agents having at least five carbon atoms. The extrudate that isformed in the present invention from at least one alkenyl aromaticpolymer, the at least one VOC blowing agent, and acetone comprises lessthan about 2.7 wt. % VOC blowing agent.

[0022] It is contemplated that the blowing agent mixture of the presentinvention may further comprise carbon dioxide (CO₂). In one embodimentof the present invention, carbon dioxide is in the liquid state beforeinjection into the extruder. Carbon dioxide in gaseous state orsupercritical fluid state may, however, also be used in the presentinvention. It is desirable to add carbon dioxide because of its highfoaming potential. Only limited amounts of carbon dioxide, however, areintroduced so as to prevent or inhibit uncontrollable foaming, formationof poor quality products (e.g., products with excessive and unwantedcorrugation), and/or significant loss of equipment productivity. It isdesirable that if carbon dioxide is used, then it is maintained at acertain amount in the system to achieve better final sheet densitycontrol.

[0023] It is contemplated that other non-VOC blowing agents such asmethyl acetate and/or water may be added to the blowing agent mixture ofthe present invention.

[0024] The blowing agent mixture of the present invention preferablyconsists essentially of at least one VOC blowing agent (n-butane,iso-butane, propane and combinations thereof), acetone and carbondioxide. The blowing agent mixture of the present invention morepreferably consists of at least one VOC blowing agent (n-butane,iso-butane, propane and combinations thereof), acetone and carbondioxide.

[0025] The extrudate that is formed from at least one alkenyl aromaticpolymer, the at least one VOC blowing agent, and acetone comprises lessthan about 2.7 wt. % VOC blowing agent. The extrudate generallycomprises less than about 2.4 wt. % VOC blowing agent and typicallycomprises from about 2.0 to about 2.4 wt. % of the VOC blowing agent.The extrudate generally comprises from about 0.5 to about 2.0 wt. %acetone and, more specifically, from about 0.8 to about 1.8 wt. %acetone. In one embodiment, the extrudate comprises from about 2.0 toabout 2.4 wt. % VOC blowing agent, from about 0.5 to about 2.0 wt. %acetone, and from about 0.3 to about 1.0 wt. % carbon dioxide.

[0026] The total amount of the blowing agent mixture used depends onconditions such as extrusion-process conditions at mixing, the blowingagent mixture being used, the composition of the extrudate, and thedesired density of the foamed article. The extrudate is defined hereinas including the blowing agent mixture, at least one alkenyl aromaticpolymer, and any additives. For a foam sheet having an extruded densityof from about 2 to about 7 lbs/ft³, the extrudate typically comprisesfrom about 7 to about 3 wt. % of the blowing agent mixture. Theextrudate more typically comprises from about 6 to about 3.5 wt. % ofthe blowing agent mixture.

[0027] Other Additives

[0028] A nucleating agent or combination of such agents may be employedin the present invention for advantages, such as its capability forregulating cell formation and morphology. A nucleating agent, or cellsize control agent, may be any conventional or useful nucleatingagent(s). The amount of nucleating agent used depends upon the desiredcell size, the selected blowing agent mixture, and the desired foamdensity. The nucleating agent is generally added in amounts from about0.1 to about 2.0 wt. % of the alkenyl aromatic polymer composition.

[0029] Some contemplated nucleating agents include inorganic materials(in small particulate form), such as clay, talc, silica, anddiatomaceous earth. Other contemplated nucleating agents include organicnucleating agents that decompose or react at the heating temperaturewithin an extruder to evolve gases such as carbon dioxide and/ornitrogen. It is contemplated that mixtures of different nucleatingagents may be added in the present invention. Some more desirablenucleating agents include talc, crystalline silica, and a stoichiometricmixture of citric acid and sodium bicarbonate (the stoichiometricmixture having a 1 to 100 percent concentration where the carrier is asuitable polymer such as polystyrene). Talc may be added in a carrier orin a powder form.

[0030] If desired, fillers, colorants, light and heat stabilizers,anti-oxidants, acid scavengers, flame retardants, processing aids,extrusion aids and foaming additives may be used in making the alkenylaromatic polymer foam.

[0031] Processes

[0032] A conventional two-extruder tandem system with each extruderhaving a single screw may be used for extruding the foam article of thepresent invention. Alternatively, a two-extruder tandem system in whichthe primary extruder is a twin screw, and the secondary extruder is asingle screw may be used for extruding the foam article of the presentinvention. A single extruder with proper cooling may also be employed inthe present invention.

[0033] According to one process of the present invention, alkenylaromatic polymer (e.g., polystyrene resin) pellets are admixed with anucleating agent, such as talc. These materials are continuously fedinto a hopper of an extruder. The feed mixture is conveyed forward by ascrew within a barrel of the extruder as the mixture is mixed,compressed, heated, and converted to molten form. The conversion tomolten form occurs prior to reaching the injection zone where theblowing agent mixture of at least one VOC blowing agent (n-butane,iso-butane, propane and combinations thereof) and acetone is added. Theblowing agent mixture of the present invention may be injected into thepolystyrene composition at a point where the polystyrene is in a meltstate (i.e., beyond the feed zone).

[0034] After injecting the blowing agent mixture, the blowing agentmixture and the at least one polystyrene resin is continuously mixed atpressures to ensure a homogeneous solution of the resin and the blowingagent mixture. The molten mixture is then conveyed into a cooling zonewhere additional mixing takes place. The mixture is then extrudedthrough a die into a lower pressure zone, such as atmospheric pressure.

[0035] According to one embodiment, a two-extruder tandem system 10 ofthe FIGURE may be used for extruding an alkenyl aromatic polymer foamarticle (e.g., a polystyrene foam article) of the present invention.Polystyrene resin pellets are mixed with at least one additive (e.g., anucleating agent) to form a feed mixture which is fed continuously intoa hopper 11 of a primary extruder 13. The feed mixture is conveyedforward by a helical screw within a barrel of the extruder as the feedmixture is mixed, compressed, heated and melted prior to reaching theblowing-agent-injection zone. The blowing agent mixture is added atpoint 15. Thus, the blowing agent mixture of the present invention isinjected into the polystyrene/additives mixture (feed mixture) at apoint beyond the feed zone where the polystyrene is melted. It iscontemplated that the blowing agent mixture may be injected at otherlocations, including a secondary extruder.

[0036] Following injection of the blowing agent mixture, the mixture iscontinuously mixed in the primary extruder 13. The exit pressure of theprimary extruder 13 is generally from about 2,000 to about 5,000 psi.The temperature of the primary extruder 13 is generally from about 420to about 500° F. The mixture is subsequently passed, at a high enoughpressure that the blowing agent mixture remains in solution, through ahollow adapter section 17 into a cooled secondary tandem extruder 19.The molten mixture is passed along the length of the cooled secondaryextruder at low shear where cooling and additional mixing occur. Theextruder exit pressure of the secondary extruder 19 is generally fromabout 1,500 to about 3,500 psi. The temperature of the extrudate fromthe secondary extruder 19 is generally from about 250 to about 340° F.

[0037] In general, the temperature of the primary extruder should besufficient to melt the polystyrene resin and any additives, and topromote efficient mixing. The temperature and pressure in the secondaryextruder should be sufficient to keep the polystyrene resin and theblowing agent mixture as a homogeneous solution. The mixture is thenexpressed through an annular die 21, though a die of a differentconfiguration, such as a flat die, may also be used. The foamablepolystyrene resin is extruded through the annular die 21 in the form ofan elongated bubble or tube 23. The foamable polystyrene resin in theFIGURE is expanded and drawn over a cylindrical surface of a cooling andsizing drum 25, and slit to form sheet stock 27. The sheet stock 27 istaken up on one or more winding reels 29.

[0038] Foamed Product

[0039] The alkenyl aromatic polymer (e.g., polystyrene) foam structureof the present invention has an extruded density less than about 7lbs/ft³ and a basis weight of less than about 20 grams per 100 in². Thealkenyl aromatic polymer (e.g., polystyrene) foam structure typicallyhas an extruded density of from about 3 to about 7 lbs/ft³. The alkenylaromatic polymer foam structure of the present invention more typicallyhas a basis weight of less than about 15 grams per 100 in². Morespecifically, the alkenyl aromatic polymer foam structure of the presentinvention has a basis weight of from about 6 to about 18 grams per 100in². The alkenyl aromatic polymer foam structure generally has athermoformed density of from about 1 to about 6 lbs/ft³ and, moretypically, from about 1.5 to about 4 lbs/ft³.

[0040] It is also highly desirable for the alkenyl aromatic polymer foamstructures to be “dimensionally stable” in the short term. Dimensionallystable as defined herein is when the gauge of the foam does not deviatemore than about 15% (i.e., the foam does not either shrink more thanabout 15% or expand more than about 15%) from the gauge of the alkenylaromatic polymer foam at the time of the production. The initialextruded gauge of the alkenyl aromatic polymer foam at the time of theproduction refers to its gauge at about 5 minutes after the foam exitsthe die. This measurement is used in determining the initial extrudedgauge of the foam. To have a dimensionally stable product, the gauge ofthe alkenyl aromatic polymer foam is then typically measured after about30 to 45 minutes (referred to as aged gauge) and compared to its initialextruded gauge. It is desirable that the aged gauge of the foam does notdeviate more than about 10% from its initial extruded gauge. If the foamdoes not have short-term dimensional stability, then the alkenylaromatic polymeric sheet may have difficulties in winding upon itself.

[0041] It is desirable to produce a alkenyl aromatic polymer foam thatwill not reduce equipment productivity or affect the quality of theproduct (e.g., minimized or no corrugation). The alkenyl aromaticpolymer foam structures may be used for a variety of applications,including supermarket trays, fast food containers, bowls and plates.

EXAMPLES

[0042] Various blowing agents and blowing agent mixtures were testedwith the results shown below in Table 1. Specifically, several foamswere made from comparative blowing agents or comparative blowing agentmixtures, and inventive blowing agent mixtures. It should be noted thatin the various examples reported in Table 1, the hardware was the sameand operated in exactly the same way; the only variable was the blowingagent or blowing agent mixture, except as noted in the descriptionbelow. All of the inventive blowing agent mixtures consisted of (a)iso-butane (“i-butane”) and acetone, (b) iso-butane, acetone and carbondioxide, (c) n-butane and acetone, or (d) n-butane, acetone and carbondioxide. The comparative blowing agents or comparative blowing agentmixtures consisted of (a) iso-pentane (“i-pentane”) and carbon dioxide,(b) iso-pentane, carbon dioxide and acetone, (c) n-butane, or (d)n-butane of 3.73 wt. % of the extrudate, carbon dioxide and acetone.

[0043] Each of the foams was made of polystyrene on a tandem extrusionline employing 2.5 inch and 3.5 inch single-screw extruders and used oneport in the primary extruder for injecting compressed fluids. Thepolystyrene resin was 50 wt. % of Dow's general purpose polystyreneSTYRON™ 685D resin and 50 wt. % reclaimed polystyrene that was derivedfrom Dow's STYRON™ 685D resin. The reclaimed material also includedtalc. In addition to the blowing agent or blowing agent mixture, and thepolystyrene mixture, talc was added in varying amounts (0.4 to 1.8 wt.%) to make the foams.

[0044] The polystyrene resin was introduced along with the talc(nucleating agent) at the feed throat of the primary extruder where itwas melted and pressurized to between 2300 psi and 3890 psi at atemperature of approximately 465° F. The blowing agent mixture wasintroduced into the melt after a first metering zone of the primaryextruder where it was thoroughly mixed with the melt and conveyed to thesecondary extruder. The secondary extruder cooled the melt to between279° F. and 305° F. at an exit pressure of between 1220 psi and 1760psi, and conveyed the melt to an annular die.

[0045] The extruded foam tube was stabilized over a mandrel, and thenslit to form a sheet. The extruded polystyrene foam sheets were wound upinto rolls after determining their extruded sheet basis weight, extrudedsheet density, initial extruded gauge and gauge loss after 30 or 45minutes. The density was measured in accordance with an ASTM D 1622-98.The gauge loss was determined by the following equation:(gauge_(initial)−gauge_(aged))/(gauge_(initial)), wherein the “gaugeinitial” is the first measured gauge after about 5 minutes and the “agedgauge” is the second measured gauge after about 30 to 45 minutes. Theextruded polystyrene foam sheets had lower basis weights that rangedfrom 8.9 to 10 g/100 in² (Comparative Examples 3-6, 18, and 19, andInventive Examples 7-9 and 20). The extruded polystyrene foam sheetsalso had higher basis weights that ranged from 14.6 to 15 g/100 in²(Comparative Examples 1, 2, and 12, and Inventive Examples 10, 11, and13-17).

[0046] For Comparative Examples 1-6, the sheets of polystyrene wereformed into a roll and allowed to age at room temperature for a durationof 3 to 7 days. After aging, the polystyrene foam sheets werethermoformed into supermarket trays. Various properties of thesupermarket trays were determined 7 days after being thermoformed. Thesedetermined properties included their thermoformed part density, partbasis weight, flexural rigidity, and flexural rigidity to basis weightratio. The flexural rigidity of all of the examples was determined inaccordance with Gibson, Lorna J. and Ashby, Michael F., “CellularSolids; Structure and Properties”, Cambridge University Press, SecondEdition, 1997, page 372, ISBN 0 52149911 9.

[0047] For Inventive Examples 7-9, 13, 14 and 20, and ComparativeExamples 12, 18 and 19, the polystyrene foam sheets were formed into aroll and allowed to age at room temperature for a duration of 3 to 7days. After aging, the polystyrene foam sheets were thermoformed intosupermarket trays. Various properties of the supermarket trays weredetermined 7 to 14 days after being thermoformed. These determinedproperties included their thermoformed part density, part basis weight,flexural rigidity, and flexural rigidity to basis weight ratio.

[0048] For Inventive Examples 10, 11, and 15-17, the foam sheets ofpolystyrene were formed into a roll and allowed to age at roomtemperature for a duration of 32 to 35 days. After aging, thepolystyrene foam sheets were thermoformed into supermarket trays.Various properties of the supermarket trays were determined seven daysafter being thermoformed. These determined properties included theirthermoformed part density, part basis weight, flexural rigidity, andflexural rigidity to basis weight ratio. TABLE 1 Extruded VOC SheetExtruded Blowing VOC Basis Sheet Agent BA CO₂ Acetone Weight DensityExamples (BA) (wt. %) (wt. %) (wt. %) (g/100 in²) (lbs/ft³) Comparativei-pentane ^((b)) 4.20 0.55 0 14.7 4.11 Ex 1 Comparative i-pentane 3.300.54 0.82 14.4 4.15 Ex 2 Comparative i-pentane 4.78 0.49 0 8.9 3.84 Ex 3Comparative i-pentane 3.85 0.63 0.96 9.2 3.93 Ex 4 Comparative i-pentane2.26 0.63 1.56 9.9 5.79 Ex 5 Comparative i-pentane 1.75 0.69 2.03 10.05.59 Ex 6 Inventive i-Butane ^((c)) 2.62 0.69 1.80 9.2 3.72 Ex 7Inventive i-Butane 2.30 0.61 0.73 9.9 4.76 Ex 8 Inventive i-Butane 1.940.76 1.08 9.8 4.66 Ex 9 Inventive i-Butane 2.11 0 1.78 14.8 5.62 Ex 10Inventive i-Butane 2.18 0.48 1.83 15.0 3.88 Ex 11 Comparative n-Butane3.76 0 0 14.6 3.96 Ex 12 Inventive n-Butane 2.75 0 0.88 14.7 4.33 Ex 13Inventive n-Butane 2.70 0.32 0.92 14.6 3.88 Ex 14 Inventive n-Butane2.15 0 1.80 14.7 4.90 Ex 15 Inventive n-Butane 2.15 0.40 1.78 14.7 4.05Ex 16 Inventive n-Butane 2.01 0.68 1.36 15.0 3.88 Ex 17 Comparativen-Butane 5.02 0 0 9.4 3.10 Ex 18 Comparative n-Butane 3.73 0.43 1.27 8.93.13 Ex 19 Inventive n-Butane 2.48 0.68 1.65 9.0 3.60 Ex 20 GaugeInitial Loss Thermoformed Part Basis Flex Extruded after part WeightFlexural Rigidity Gauge 45 min. density (BW) Rigidity to BW Examples(mils) (%) (lbs/ft³) (g/100 in²) (lbs. in²) ratio Comparative 136 <2%^((a)) 2.14 12.7 4.42 0.35 Ex 1 Comparative 132 <2% ^((a)) 2.21 12.73.94 0.31 Ex 2 Comparative 88 2% 1.78 8.1 1.73 0.21 Ex 3 Comparative 896% 1.68 7.7 1.98 0.26 Ex 4 Comparative 65 19% 2.70 9.0 1.52 0.17 Ex 5Comparative 68 18% 2.73 9.2 1.47 0.16 Ex 6 Inventive 94 1% 1.91 8.5 2.230.26 Ex 7 Inventive 79 2% 2.66 9.0 1.51 0.17 Ex 8 Inventive 80 6% 2.438.8 1.61 0.18 Ex 9 Inventive 100 0% ^((a)) 2.50 14.4 5.99 0.42 Ex 10Inventive 147 0% ^((a)) 2.00 14.7 7.85 0.53 Ex 11 Comparative 140 0%^((a)) 1.50 12.2 5.70 0.47 Ex 12 Inventive 129 0% ^((a)) 1.55 12.6 5.870.47 Ex 13 Inventive 143 0% ^((a)) 1.47 12.0 6.24 0.52 Ex 14 Inventive114 0% ^((a)) 2.37 12.7 5.09 0.40 Ex 15 Inventive 138 0% ^((a)) 2.1713.2 5.87 0.44 Ex 16 Inventive 147 0% ^((a)) 1.83 13.1 7.04 0.54 Ex 17Comparative 115 0% 1.42 7.6 2.12 0.28 Ex 18 Comparative 108 2% 1.51 7.52.05 0.27 Ex 19 Inventive 95 3% 2.18 9.2 2.60 0.28 Ex 20

[0049] As shown in Table 1, when the blowing agent mixture includediso-pentane as the sole VOC blowing agent (see Comparative Examples1-6), the resulting foam was not dimensionally stable at lower weightpercentages of iso-pentane. Compare Comparative Examples 5 and 6 (2.26and 1.75 wt. % iso-pentane with gauge losses of 19 and 18%,respectively) with Comparative Examples 1-4 (between 3.30 and 4.78 wt. %iso-pentane with gauge losses below 6%). Thus, as shown in ComparativeExamples 1-6, when the iso-pentane of the blowing agent mixture wasreplaced by an increased amount of acetone, the extruded polystyrenefoam sheet became more susceptible to short term dimensional stabilityproblems (reduction of the sheet thickness after extrusion). This wasespecially prevalent at lower extruded sheet basis weights (8.9-10.0g/100 in²) of Comparative Examples 3-6.

[0050] This, however, was not the case using a blowing agent mixturethat comprised iso-butane or n-butane with at least acetone. As shown inInventive Examples 7-11, a blowing agent mixture comprising from 1.94 to2.62 wt. % iso-butane with at least acetone surprisingly produced gaugelosses of only from between 0 and 6%. The highest gauge loss ofInventive Examples 7-11 (6% in Inventive Example 9) was formed using ablowing agent mixture comprising 1.94 wt. % iso-butane, which was lowerthan the iso-butane used in the blowing agent mixtures of InventiveExamples 7, 8, 10, and 11. Similarly, as shown in Inventive Examples13-17 and 20, a blowing agent mixture comprising from 2.01 to 2.75 wt. %n-butane with at least acetone surprisingly produced gauge losses ofonly from between 0 and 3%. Additionally, the Inventive Examples did notsacrifice flexural rigidity when compared to the Comparative Examples.

[0051] Thus, Table 1 shows that when the comparative blowing agentmixtures comprised of at least iso-pentane and acetone are compared tothe inventive blowing agent mixtures comprised of at least either (a)iso-butane and acetone or (b) n-butane and acetone at lower VOC weightpercentages, the dimensional stability of the inventive foams isunexpectedly much improved than the comparative foams at similarly lowerVOC weight percentages.

[0052] While particular embodiments and applications of the presentinvention have been illustrated and described, it is to be understoodthat the invention is not limited to the precise construction andcompositions disclosed herein and that various modifications, changes,and variations may be apparent from the foregoing descriptions withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

What is claimed is:
 1. A method for producing an alkenyl aromaticpolymer foam structure comprising: melting at least one alkenyl aromaticpolymer resin; dissolving an effective amount of blowing agent mixturein the at least one alkenyl aromatic polymer, the blowing agent mixturecomprising at least one volatile organic compound (VOC) blowing agentand acetone, the majority by mole percent of the VOC blowing agent beingselected from n-butane, iso-butane, propane and combinations thereof;forming an extrudate comprising the at least one alkenyl aromaticpolymer resin, the at least one VOC blowing agent, and acetone, theextrudate comprising less than about 2.7 wt. % of the at least one VOCblowing agent, transferring the extrudate to an expansion zone; andpermitting the extrudate to expand in the expansion zone to produce thealkenyl aromatic polymer foam structure, the alkenyl aromatic polymerfoam structure having an extruded density less than about 7 lbs/ft³ anda basis weight of less than about 20 grams per 100 in².
 2. The method ofclaim 1, wherein the blowing agent mixture further comprises carbondioxide.
 3. The method of claim 1, wherein the blowing agent mixturefurther comprises carbon dioxide and consists essentially of the atleast one VOC blowing agent being selected from n-butane, iso-butane,propane and combinations thereof, acetone and carbon dioxide.
 4. Themethod of claim 1, wherein the blowing agent mixture further comprisesmethyl acetate and/or water.
 5. The method of claim 1, wherein theextrudate comprises less than about 2.4 wt. % VOC blowing agent.
 6. Themethod of claim 5, wherein the extrudate comprises from about 2.0 toabout 2.4 wt. % VOC blowing agent.
 7. The method of claim 1, wherein theextrudate comprises from about 0.5 to about 2.0 wt. % acetone.
 8. Themethod of claim 7, wherein the extrudate comprises from about 0.8 toabout 1.8 wt. % acetone.
 9. The method of claim 1, wherein the blowingagent mixture further comprises carbon dioxide and the extrudatecomprises from about 2.0 to about 2.4 wt % VOC blowing agent, from about0.5 to about 2.0 wt. % acetone, and from about 0.3 to about 1.0 wt. %carbon dioxide.
 10. The method of claim 1, wherein the extrudatecomprises from about 3 to about 7 wt. % of the blowing agent mixture.11. The method of claim 1, wherein the blowing agent mixture is formedin the substantial absence of VOC blowing agents having at least fivecarbons atoms.
 12. The method of claim 11, wherein the blowing agentmixture is formed in the absence of VOC blowing agents having at leastfive carbons atoms.
 13. The method of claim 1, wherein the alkenylaromatic polymer foam structure has an extruded density of from about 3to about 7 lbs/ft³.
 14. The method of claim 1, wherein the at least onealkenyl aromatic polymer foam structure is a homopolymer of styrene,copolymers of styrene, or rubber-toughened polystyrene.
 15. The methodof claim 1, wherein the polystyrene foam structure has a basis weight ofless than about 15 grams per 100 in².
 16. A alkenyl aromatic polymerfoam structure prepared by the process comprising: melting at least onealkenyl aromatic polymer resin, dissolving an effective amount ofblowing agent mixture in the at least one alkenyl aromatic polymer, theblowing agent mixture comprising at least one volatile organic compound(VOC) blowing agent and acetone, the majority by mole percent of the VOCblowing agent being selected from n-butane, iso-butane, propane andcombinations thereof, forming an extrudate comprising the at least onealkenyl aromatic polymer resin, the at least one VOC blowing agent, andacetone, the extrudate comprising less than about 2.7 wt. % of the atleast one VOC blowing agent, transferring the extrudate to an expansionzone; and permitting the extrudate to expand in the expansion zone toproduce the alkenyl aromatic polymer foam structure, wherein the alkenylaromatic polymer foam structure has an extruded density less than about7 lbs/ft³ and a basis weight of less than about 20 grams per 100 in².17. The foam structure of claim 16, wherein the blowing agent mixturefurther comprises carbon dioxide.
 18. The foam structure of claim 16,wherein the blowing agent mixture further comprises carbon dioxide andconsists essentially of the at least one VOC blowing agent beingselected from n-butane, iso-butane, propane and combinations thereof,acetone and carbon dioxide.
 19. The foam structure of claim 16, whereinthe extrudate comprises less than about 2.4 wt. % VOC blowing agent. 20.The foam structure of claim 16, wherein the blowing agent mixturefurther comprises carbon dioxide and the extrudate comprises from about2.0 to about 2.4 wt. % VOC blowing agent, from about 0.5 to about 2.0wt. % acetone, and from about 0.3 to about 1.0 wt. % carbon dioxide. 21.The foam structure of claim 16, wherein the blowing agent mixture isformed in the substantial absence of VOC blowing agents having at leastfive carbons atoms.
 22. The foam structure of claim 21, wherein theblowing agent mixture is formed in the absence of VOC blowing agentshaving at least five carbons atoms.
 23. A method for producing analkenyl aromatic polymer foam structure comprising: melting at least onealkenyl aromatic polymer resin, dissolving an effective amount ofblowing agent mixture in the at least one alkenyl aromatic polymer, theblowing agent mixture consists essentially of at least one volatileorganic compound (VOC) blowing agent and acetone, the VOC blowing agentbeing selected from n-butane, iso-butane, propane and combinationsthereof, forming an extrudate comprising the at least one alkenylaromatic polymer resin, the at least one VOC blowing agent, and acetone,the extrudate comprising less than about 2.7 wt. % of the at least oneVOC blowing agent; transferring the extrudate to an expansion zone; andpermitting the extrudate to expand in the expansion zone to produce thealkenyl aromatic polymer foam structure, the alkenyl aromatic polymerfoam structure having an extruded density less than about 7 lbs/ft³ anda basis weight of less than about 20 grams per 100 in².
 24. The methodof claim 23, wherein the extrudate comprises less than about 2.4 wt. %VOC blowing agent.
 25. The method of claim 23, wherein the extrudatecomprises from about 2.0 to about 2.4 wt. % VOC blowing agent.
 26. Themethod of claim 23, wherein the extrudate comprises from about 0.5 toabout 2.0 wt. % acetone.
 27. The method of claim 23, wherein the blowingagent mixture is formed in the absence of VOC blowing agents having atleast five carbons atoms.
 28. The method of claim 23, wherein the atleast one alkenyl aromatic polymer foam structure is a homopolymer ofstyrene, copolymers of styrene, or rubber-toughened polystyrene.
 29. Amethod for producing an alkenyl aromatic polymer foam structurecomprising: melting at least one alkenyl aromatic polymer resin;dissolving an effective amount of blowing agent mixture in the at leastone alkenyl aromatic polymer, the blowing agent mixture consistsessentially of at least one volatile organic compound (VOC) blowingagent, acetone and carbon dioxide, the VOC blowing agent being selectedfrom n-butane, iso-butane, propane and combinations thereof, forming anextrudate comprising the at least one alkenyl aromatic polymer resin,the at least one VOC blowing agent, acetone and carbon dioxide, theextrudate comprising less than about 2.7 wt. % of the at least one VOCblowing agent, transferring the extrudate to an expansion zone; andpermitting the extrudate to expand in the expansion zone to produce thealkenyl aromatic polymer foam structure, the alkenyl aromatic polymerfoam structure having an extruded density less than about 7 lbs/ft³ anda basis weight of less than about 20 grams per 100 in².
 30. The methodof claim 29, wherein the extrudate comprises less than about 2.4 wt. %VOC blowing agent.
 31. The method of claim 29, wherein the extrudatecomprises from about 2.0 to about 2.4 wt. % VOC blowing agent.
 32. Themethod of claim 29, wherein the extrudate comprises from about 0.5 toabout 2.0 wt. % acetone.
 33. The method of claim 29, wherein the blowingagent mixture is formed in the absence of VOC blowing agents having atleast five carbons atoms.
 34. The method of claim 29, wherein the atleast one alkenyl aromatic polymer foam structure is a homopolymer ofstyrene, copolymers of styrene, or rubber-toughened polystyrene.
 35. Amethod for producing an alkenyl aromatic polymer foam structurecomprising: melting at least one alkenyl aromatic polymer resin,dissolving an effective amount of blowing agent mixture in the at leastone alkenyl aromatic polymer, the blowing agent mixture consistsessentially of at least one volatile organic compound (VOC) blowingagent, acetone and carbon dioxide, the VOC blowing agent being selectedfrom n-butane, iso-butane, propane and combinations thereof; forming anextrudate comprising the at least one alkenyl aromatic polymer resin,the at least one VOC blowing agent, acetone and carbon dioxide, theextrudate comprising less than about 2.4 wt. % of the at least one VOCblowing agent; transferring the extrudate to an expansion zone; andpermitting the extrudate to expand in the expansion zone to produce thealkenyl aromatic polymer foam structure, the alkenyl aromatic polymerfoam structure having an extruded density less than about 7 lbs/ft³ anda basis weight of less than about 20 grams per 100 in².
 36. The methodof claim 35, wherein the extrudate comprises from about 2.0 to about 2.4wt. % VOC blowing agent.
 37. The method of claim 35, wherein theextrudate comprises from about 0.5 to about 2.0 wt. % acetone.
 38. Themethod of claim 35, wherein the blowing agent mixture is formed in theabsence of VOC blowing agents having at least five carbons atoms. 39.The method of claim 35, wherein the at least one alkenyl aromaticpolymer foam structure is a homopolymer of styrene, copolymers ofstyrene, or rubber-toughened polystyrene.
 40. The method of claim 35,wherein the blowing agent further comprises carbon dioxide.
 41. A methodfor producing a thermoformed alkenyl aromatic polymer articlecomprising: melting at least one alkenyl aromatic polymer resin,dissolving an effective amount of blowing agent mixture in the at leastone alkenyl aromatic polymer, the blowing agent mixture comprising atleast one volatile organic compound (VOC) blowing agent and acetone, themajority by mole percent of the VOC blowing agent being selected fromn-butane, iso-butane, propane and combinations thereof, forming anextrudate comprising the at least one alkenyl aromatic polymer resin,the at least one VOC blowing agent, and acetone, the extrudatecomprising less than about 2.7 wt. % of the at least one VOC blowingagent, transferring the extrudate to an expansion zone; permitting theextrudate to expand in the expansion zone to produce the alkenylaromatic polymer foam structure, the alkenyl aromatic polymer foamstructure having a basis weight of less than about 20 grams per 100 in²,and thermoforming the alkenyl aromatic polymer foam structure into anarticle, the thermoformed density of the article being less than 6lbs/ft³.
 42. The method of claim 41, wherein the thermoformed article isa homopolymer of styrene, copolymers of styrene, or rubber-toughenedpolystyrene.
 43. The method of claim 41, wherein the blowing agentmixture further comprises carbon dioxide.